For example, a monopole antenna as shown in FIG. 16A is disclosed in “B-77: BROADBAND CHARACTERISTICS OF SEMI-CIRCULAR ANTENNA COMBINED WITH LINEAR ELEMENT”, Taisuke Ihara, Makoto Kijima and Koichi Tsunekawa, pp77 General Convention of The Institute of Electronics, Information and Communication Engineers, 1996 (hereinafter referred to as “non-patent document 1”). In FIG. 16A, a semicircular element 1010 is erected vertically to a earth plate 1011, and the closest point of the arc of the element 1010 to the earth plate 1011 serves as a feed portion 1012. The non-patent document 1 discloses that a frequency fL at which the radius of the circle corresponds approximately to a quarter wavelength is the lower limit. Besides, it describes an example in which as shown in FIG. 16B, an element 1013 where a cut-out portion is provided for the element 1010 shown in FIG. 16A is erected vertically to the earth plate 1011, and that VSWR (Voltage Standing Wave Ratio) characteristics of the monopole antenna of FIG. 16A and the monopole antenna of FIG. 16B are almost identical to each other. Further, it also discloses an example in which as shown in FIG. 16C, an element 1014 in which an element 1014a, which resonates at a frequency lower than fL and has a meander monopole structure, is connected to an element with a cut-out portion as shown in FIG. 16B is erected vertically to the earth plate 1011. Incidentally, the element 1014a is disposed to be accommodated in the cut-out portion. By the element 1014a, the antenna also resonates at a frequency lower than fL, and multi-resonance is realized, however, the VSWR characteristic in a frequency range lower than fL is poor, and sufficient characteristics for use in a dual band antenna are not achieved.
Besides, U.S. Pat. No. 6,515,626 discloses a microstrip patch antenna 1100 as shown in FIG. 17. The microstrip patch antenna 1100 is such that a ground plane 1140, a microstrip patch 1120, and a triangular pad (feed conductor) 1130 connected to the microstrip patch 1120 are formed of conductive metal on a dielectric substrate 1110. Incidentally, the microstrip patch 1120 is fed from a feed point 1150 through the triangular pad 1130 as a feed conductor. Although not shown, from the operation principle of the microstrip antenna, the microstrip patch antenna 1100 as shown in FIG. 17 is not suitably operated unless the ground is disposed opposite to the dielectric substrate 1110. Besides, since the area of the ground plane 1140 is very small, it is not conceivable that the ground plane functions as a radiant element. Further, in the microstrip antenna, a current flowing in the radiation conductor is not a direct radiation source, and in FIG. 17, a current flowing in the triangular pad 1130 and the microstrip patch 1120 does not serve as a direct radiation source. Besides, a reception frequency bandwidth of the microstrip patch antenna 1100 disclosed in this publication is as narrow as 200 MHz with respect to the center frequency of 1.8 GHz, the triangular pad 1130 does not function as the radiation conductor, and it is conceivable that the microstrip patch 1120 is a radiation conductor of a single frequency (1.8 GHz). As stated above, the microstrip patch antenna 1100 shown in FIG. 17 is a microstrip antenna and is not a monopole antenna in which a current flowing in the radiation conductor contributes to radiation. Besides, it is not a traveling-wave antenna in which the wide bandwidth is realized by continuously changing a current path flowing in a radiation conductor. Further, since the reception frequency range is single, it is not a dual band antenna.
As stated above, although there are various antennas up to now, since the conductor in the conventional monopole antenna disclosed in the non-patent document 1 is erected vertically to the earth plate, the size of the antenna becomes large.
Besides, in the antenna disclosed in the non-patent document 1, as set forth above, although multi-resonance is realized in plural frequency ranges, antenna characteristics that are presently demanded as the dual band antenna are not obtained.
Further, with respect to the microstrip antenna disclosed in U.S. Pat. No. 6,515,626, although the shape appears to be such that both the triangular pad and the microstrip patch contribute to radiation, the triangular pad does not serve as the radiation conductor, but is merely the feed conductor. Thus, this antenna is the antenna in which the reception frequency range is single, and is not the dual band antenna.